GAA  Grup d'Astronomia i Astrofísica
http://hdl.handle.net/2117/1136
20170225T11:55:25Z

Multistep derivativefree preconditioned Newton method for solving systems of nonlinear equations
http://hdl.handle.net/2117/101565
Multistep derivativefree preconditioned Newton method for solving systems of nonlinear equations
Ahmad, Fayyaz
Preconditioning of systems of nonlinear equations modifies the associated Jacobian and provides rapid convergence. The preconditioners are introduced in a way that they do not affect the convergence order of parent iterative method. The multistep derivativefree iterative method consists of a base method and multistep part. In the base method, the Jacobian of the system of nonlinear equation is approximated by finite difference operator and preconditioners add an extra term to modify it. The inversion of modified finite difference operator is avoided by computing LU factors. Once we have LU factors, we repeatedly use them to solve lower and upper triangular systems in the multistep part to enhance the convergence order. The convergence order of mstep Newton iterative method is m + 1. The claimed convergence orders are verified by computing the computational order of convergence and numerical simulations clearly show that the good selection of preconditioning provides numerical stability, accuracy and rapid convergence.
20170224T15:53:27Z
Ahmad, Fayyaz
Preconditioning of systems of nonlinear equations modifies the associated Jacobian and provides rapid convergence. The preconditioners are introduced in a way that they do not affect the convergence order of parent iterative method. The multistep derivativefree iterative method consists of a base method and multistep part. In the base method, the Jacobian of the system of nonlinear equation is approximated by finite difference operator and preconditioners add an extra term to modify it. The inversion of modified finite difference operator is avoided by computing LU factors. Once we have LU factors, we repeatedly use them to solve lower and upper triangular systems in the multistep part to enhance the convergence order. The convergence order of mstep Newton iterative method is m + 1. The claimed convergence orders are verified by computing the computational order of convergence and numerical simulations clearly show that the good selection of preconditioning provides numerical stability, accuracy and rapid convergence.

The evolution of white dwarfs resulting from heliumenhanced, lowmetallicity progenitor stars
http://hdl.handle.net/2117/101528
The evolution of white dwarfs resulting from heliumenhanced, lowmetallicity progenitor stars
Althaus, Leandro G.; de Geronimo, Francisco; Córsico, Alejandro H.; Torres Gil, Santiago; GarcíaBerro Montilla, Enrique
Context. Some globular clusters host multiple stellar populations with different chemical abundance patterns. This is particularly true for ¿ Centauri, which shows clear evidence of a heliumenriched subpopulation characterized by a helium abundance as high as Y = 0.4
Aims. We present a whole and consistent set of evolutionary tracks from the ZAMS to the white dwarf stage that is appropriate for the study of the formation and evolution of white dwarfs resulting from the evolution of heliumrich progenitors.
Methods. We derived white dwarf sequences from progenitors with stellar mass ranging from 0.60 to 2.0 M¿ and for an initial helium abundance of Y = 0.4. We adopted two values of metallicity: Z = 0.001 and Z = 0.0005.
Results. We explored different issues of white dwarf evolution and their heliumrich progenitors. In particular, the final mass of the remnants, the role of overshooting during the thermally pulsing phase, and the cooling of the resulting white dwarfs differ markedly from the evolutionary predictions of progenitor stars with the standard initial helium abundance. Finally, the pulsational properties of the resulting white dwarfs are also explored.
Conclusions. We find that, for the range of initial masses explored in this paper, the final mass of the heliumrich progenitors is markedly higher than the final mass expected from progenitors with the usual helium abundance. We also find that progenitors with initial mass lower than M ¿ 0.65 M¿ evolve directly into heliumcore white dwarfs in less than 14 Gyr, and that, for larger progenitor masses, the evolution of the resulting lowmass carbonoxygen white dwarfs is dominated by residual nuclear burning. For heliumcore white dwarfs, we find that they evolve markedly faster than their counterparts coming from standard progenitors. Also, in contrast with what occurs for white dwarfs resulting from progenitors with the standard helium abundance, the impact of residual burning on the cooling time of white dwarfs is not affected by the occurrence of overshooting during the thermally pulsing phase of progenitor stars.
20170224T11:47:47Z
Althaus, Leandro G.
de Geronimo, Francisco
Córsico, Alejandro H.
Torres Gil, Santiago
GarcíaBerro Montilla, Enrique
Context. Some globular clusters host multiple stellar populations with different chemical abundance patterns. This is particularly true for ¿ Centauri, which shows clear evidence of a heliumenriched subpopulation characterized by a helium abundance as high as Y = 0.4
Aims. We present a whole and consistent set of evolutionary tracks from the ZAMS to the white dwarf stage that is appropriate for the study of the formation and evolution of white dwarfs resulting from the evolution of heliumrich progenitors.
Methods. We derived white dwarf sequences from progenitors with stellar mass ranging from 0.60 to 2.0 M¿ and for an initial helium abundance of Y = 0.4. We adopted two values of metallicity: Z = 0.001 and Z = 0.0005.
Results. We explored different issues of white dwarf evolution and their heliumrich progenitors. In particular, the final mass of the remnants, the role of overshooting during the thermally pulsing phase, and the cooling of the resulting white dwarfs differ markedly from the evolutionary predictions of progenitor stars with the standard initial helium abundance. Finally, the pulsational properties of the resulting white dwarfs are also explored.
Conclusions. We find that, for the range of initial masses explored in this paper, the final mass of the heliumrich progenitors is markedly higher than the final mass expected from progenitors with the usual helium abundance. We also find that progenitors with initial mass lower than M ¿ 0.65 M¿ evolve directly into heliumcore white dwarfs in less than 14 Gyr, and that, for larger progenitor masses, the evolution of the resulting lowmass carbonoxygen white dwarfs is dominated by residual nuclear burning. For heliumcore white dwarfs, we find that they evolve markedly faster than their counterparts coming from standard progenitors. Also, in contrast with what occurs for white dwarfs resulting from progenitors with the standard helium abundance, the impact of residual burning on the cooling time of white dwarfs is not affected by the occurrence of overshooting during the thermally pulsing phase of progenitor stars.

The white dwarf population within 40 pc of the Sun
http://hdl.handle.net/2117/101451
The white dwarf population within 40 pc of the Sun
Torres Gil, Santiago; GarcíaBerro Montilla, Enrique
© 2016 ESO. Context. The white dwarf luminosity function is an important tool to understand the properties of the solar neighborhood, like its star formation history, and its age. Aims. Here we present a population synthesis study of the white dwarf population within 40 pc from the Sun, and compare the results of this study with the properties of the observed sample. Methods. We use a stateoftheart population synthesis code based on Monte Carlo techniques, which incorporates the most recent and reliable white dwarf cooling sequences, an accurate description of the Galactic neighborhood, and a realistic treatment of all the known observational biases and selection procedures. Results. We find a good agreement between our theoretical models and the observed data. In particular, our simulations reproduce a previously unexplained feature of the bright branch of the white dwarf luminosity function, which we argue is due to a recent episode of star formation. We also derive the age of the solar neighborhood employing the position of the observed cutoff of the white dwarf luminosity function, to obtain ~8.9 ± 0.2 Gyr. Conclusions. We conclude that a detailed description of the ensemble properties of the population of white dwarfs within 40 pc of the Sun allows us to obtain interesting constraints on the history of the Solar neighborhood.
20170223T11:34:51Z
Torres Gil, Santiago
GarcíaBerro Montilla, Enrique
© 2016 ESO. Context. The white dwarf luminosity function is an important tool to understand the properties of the solar neighborhood, like its star formation history, and its age. Aims. Here we present a population synthesis study of the white dwarf population within 40 pc from the Sun, and compare the results of this study with the properties of the observed sample. Methods. We use a stateoftheart population synthesis code based on Monte Carlo techniques, which incorporates the most recent and reliable white dwarf cooling sequences, an accurate description of the Galactic neighborhood, and a realistic treatment of all the known observational biases and selection procedures. Results. We find a good agreement between our theoretical models and the observed data. In particular, our simulations reproduce a previously unexplained feature of the bright branch of the white dwarf luminosity function, which we argue is due to a recent episode of star formation. We also derive the age of the solar neighborhood employing the position of the observed cutoff of the white dwarf luminosity function, to obtain ~8.9 ± 0.2 Gyr. Conclusions. We conclude that a detailed description of the ensemble properties of the population of white dwarfs within 40 pc of the Sun allows us to obtain interesting constraints on the history of the Solar neighborhood.

The effect of 22Ne diffusion in the evolution and pulsational properties of white dwarfs with solar metallicity prgenitors
http://hdl.handle.net/2117/101446
The effect of 22Ne diffusion in the evolution and pulsational properties of white dwarfs with solar metallicity prgenitors
Camisassa, Maria E; Althaus, Leandro G.; Corsico, Alejandro H.; Vinyoles, Núria; Serenelli, Aldo M.; Isem, Jordi; Miller Bertolami, Marcelo M; GarcíaBerro Montilla, Enrique
© 2016. The American Astronomical Society. All rights reserved. Because of the large neutron excess of 22Ne, sedimentation of this isotope occurs rapidly in the interior of white dwarfs. This process releases an additional amount of energy, thus delaying the cooling times of the white dwarf. This influences the ages of different stellar populations derived using white dwarf cosmochronology. Furthermore, the overabundance of 22Ne in the inner regions of the star modifies the BruntVäisälä frequency, thus altering the pulsational properties of these stars. In this work we discuss the impact of 22Ne sedimentation in white dwarfs resulting from solar metallicity progenitors (Z = 0.02). We performed evolutionary calculations of white dwarfs with masses of 0.528, 0.576, 0.657, and 0.833 derived from full evolutionary computations of their progenitor stars, starting at the zeroAge main sequence all the way through the central hydrogen and helium burning, the thermally pulsing asymptotic giant branch (AGB), and postAGB phases. Our computations show that at low luminosities (), 22Ne sedimentation delays the cooling of white dwarfs with solar metallicity progenitors by about 1 Gyr. Additionally, we studied the consequences of 22Ne sedimentation on the pulsational properties of ZZ Ceti white dwarfs. We find that 22Ne sedimentation induces differences in the periods of these stars larger than the present observational uncertainties, particularly in more massive white dwarfs.
20170223T11:03:37Z
Camisassa, Maria E
Althaus, Leandro G.
Corsico, Alejandro H.
Vinyoles, Núria
Serenelli, Aldo M.
Isem, Jordi
Miller Bertolami, Marcelo M
GarcíaBerro Montilla, Enrique
© 2016. The American Astronomical Society. All rights reserved. Because of the large neutron excess of 22Ne, sedimentation of this isotope occurs rapidly in the interior of white dwarfs. This process releases an additional amount of energy, thus delaying the cooling times of the white dwarf. This influences the ages of different stellar populations derived using white dwarf cosmochronology. Furthermore, the overabundance of 22Ne in the inner regions of the star modifies the BruntVäisälä frequency, thus altering the pulsational properties of these stars. In this work we discuss the impact of 22Ne sedimentation in white dwarfs resulting from solar metallicity progenitors (Z = 0.02). We performed evolutionary calculations of white dwarfs with masses of 0.528, 0.576, 0.657, and 0.833 derived from full evolutionary computations of their progenitor stars, starting at the zeroAge main sequence all the way through the central hydrogen and helium burning, the thermally pulsing asymptotic giant branch (AGB), and postAGB phases. Our computations show that at low luminosities (), 22Ne sedimentation delays the cooling of white dwarfs with solar metallicity progenitors by about 1 Gyr. Additionally, we studied the consequences of 22Ne sedimentation on the pulsational properties of ZZ Ceti white dwarfs. We find that 22Ne sedimentation induces differences in the periods of these stars larger than the present observational uncertainties, particularly in more massive white dwarfs.

Frozen Jacobian iterative method for solving systems of nonlinear equations: application to nonlinear IVPs and BVPs
http://hdl.handle.net/2117/101173
Frozen Jacobian iterative method for solving systems of nonlinear equations: application to nonlinear IVPs and BVPs
Ullah, Malik Zaka; Ahmad, Fayyaz; Alshomrani, Ali Saleh; Alzahrani, A. K.; Alghamdi, Metib Said; Ahmad, Shamshad; Ahmad, Shahid
Frozen Jacobian iterative methods are of practical interest to solve the system of nonlinear equations. A
frozen Jacobian multistep iterative method is presented. We divide the multistep iterative method into two
parts namely base method and multistep part. The convergence order of the constructed frozen Jacobian
iterative method is three, and we design the base method in a way that we can maximize the convergence
order in the multistep part. In the multistep part, we utilize a single evaluation of the function, solve four
systems of lower and upper triangular systems and a second frozen Jacobian. The attained convergence
order per multistep is four. Hence, the general formula for the convergence order is 3 + 4(m  2) for
m = 2 and m is the number of multisteps. In a single instance of the iterative method, we employ only
single inversion of the Jacobian in the form of LU factors that makes the method computationally cheaper
because the LU factors are used to solve four system of lower and upper triangular systems repeatedly. The
claimed convergence order is verified by computing the computational order of convergence for a system of
nonlinear equations. The efficiency and validity of the proposed iterative method are narrated by solving
many nonlinear initial and boundary value problems.
20170217T11:47:01Z
Ullah, Malik Zaka
Ahmad, Fayyaz
Alshomrani, Ali Saleh
Alzahrani, A. K.
Alghamdi, Metib Said
Ahmad, Shamshad
Ahmad, Shahid
Frozen Jacobian iterative methods are of practical interest to solve the system of nonlinear equations. A
frozen Jacobian multistep iterative method is presented. We divide the multistep iterative method into two
parts namely base method and multistep part. The convergence order of the constructed frozen Jacobian
iterative method is three, and we design the base method in a way that we can maximize the convergence
order in the multistep part. In the multistep part, we utilize a single evaluation of the function, solve four
systems of lower and upper triangular systems and a second frozen Jacobian. The attained convergence
order per multistep is four. Hence, the general formula for the convergence order is 3 + 4(m  2) for
m = 2 and m is the number of multisteps. In a single instance of the iterative method, we employ only
single inversion of the Jacobian in the form of LU factors that makes the method computationally cheaper
because the LU factors are used to solve four system of lower and upper triangular systems repeatedly. The
claimed convergence order is verified by computing the computational order of convergence for a system of
nonlinear equations. The efficiency and validity of the proposed iterative method are narrated by solving
many nonlinear initial and boundary value problems.

Simultaneous detection of the nonlinear restoring and excitation of a forced nonlinear oscillation: an integral approach
http://hdl.handle.net/2117/101056
Simultaneous detection of the nonlinear restoring and excitation of a forced nonlinear oscillation: an integral approach
Ahmad, Fayyaz; Jang, Taek Soo; Park, Jinsoo; Sung, Hong Gun
We address in this article, how to calculate the restoring characteristic and the excitation of a nonlinear forced oscillating system. Under the assumption that the forced nonlinear oscillator has a periodic solution with period, we constructed a system of linear equations by introducing timedependent multipliers. The periodicity assumption helps simplify the system of linear equations. The stability and uniqueness are also presented for the inverse problem. Numerical testing is conducted to show the effectiveness of our presented methodology.
20170215T09:34:46Z
Ahmad, Fayyaz
Jang, Taek Soo
Park, Jinsoo
Sung, Hong Gun
We address in this article, how to calculate the restoring characteristic and the excitation of a nonlinear forced oscillating system. Under the assumption that the forced nonlinear oscillator has a periodic solution with period, we constructed a system of linear equations by introducing timedependent multipliers. The periodicity assumption helps simplify the system of linear equations. The stability and uniqueness are also presented for the inverse problem. Numerical testing is conducted to show the effectiveness of our presented methodology.

M31N 200812a  The remarkable recurrent Nova in M31: Panchromatic observations of the 2015 eruption
http://hdl.handle.net/2117/100773
M31N 200812a  The remarkable recurrent Nova in M31: Panchromatic observations of the 2015 eruption
Darnley, M. J.; Henze, Martin; Bode, M. F.; Hachisu, I.; Hernanz Carbó, Margarita; Hornoch, Kamil; Hounsell, R.; Kato, M.; Ness, JanUwe; Osborne, Julian; Page, Kim; Ribeiro, V. A.; RodríguezGil, Pablo; Shafter, A. W.; Shara, M.; Steele, I. A.; Williams, S. C.; Figueira, Joana; José Pont, Jordi; Sala Cladellas, Glòria
20170209T14:26:23Z
Darnley, M. J.
Henze, Martin
Bode, M. F.
Hachisu, I.
Hernanz Carbó, Margarita
Hornoch, Kamil
Hounsell, R.
Kato, M.
Ness, JanUwe
Osborne, Julian
Page, Kim
Ribeiro, V. A.
RodríguezGil, Pablo
Shafter, A. W.
Shara, M.
Steele, I. A.
Williams, S. C.
Figueira, Joana
José Pont, Jordi
Sala Cladellas, Glòria

Three–dimensional simulations of turbulent convective mixing in ONe and CO classical nova explosions
http://hdl.handle.net/2117/100535
Three–dimensional simulations of turbulent convective mixing in ONe and CO classical nova explosions
Casanova, J.; José Pont, Jordi; GarcíaBerro Montilla, Enrique; Shore, Steven N.
20170203T10:11:54Z
Casanova, J.
José Pont, Jordi
GarcíaBerro Montilla, Enrique
Shore, Steven N.

Coordinated analysis of two graphite grains from the CO3.0 LAP 031117 meteorite: First identification of a CO Nova graphite and a presolar iron sulfide subgrain
http://hdl.handle.net/2117/100533
Coordinated analysis of two graphite grains from the CO3.0 LAP 031117 meteorite: First identification of a CO Nova graphite and a presolar iron sulfide subgrain
Haenecour, P.; Floss, C.; José Pont, Jordi; Amari, S.; Lodders, K.; Jadhav, M.; Wang, A.; Gyngard, F.
Presolar grains constitute remnants of stars that existed before the formation of the solar system.
In addition to providing direct information on the materials from which the solar system formed, these grains provide groundtruth information for models of stellar evolution and nucleosynthesis.
Here we report the insitu identification of two unique presolar graphite grains from the primitive meteorite LaPaz Icefield 031117. Based on these two graphite grains, we estimate a bulk presolar graphite abundance of 53+7 ppm in this meteorite. One of the grains (LAP141) is characterized by an enrichment in 12C and depletions in 33,34S, and contains a small iron sulfide subgrain, representing the first unambiguous identification of presolar iron sulfide. The other grain (LAP149) is extremely 13Crich and 15Npoor, with one of the lowest 12C/13C ratios observed among presolar grains. Comparison of its isotopic compositions with new stellar
nucleosynthesis and dust condensation models indicates an origin in the ejecta of a lowmass CO nova. Grain LAP149 is the first putative nova grain that quantitatively best matches nova model
predictions, providing the first strong evidence for graphite condensation in nova ejecta. Our discovery confirms that CO nova graphite and presolar iron sulfide contributed to the original building blocks of the solar system.
20170203T09:58:18Z
Haenecour, P.
Floss, C.
José Pont, Jordi
Amari, S.
Lodders, K.
Jadhav, M.
Wang, A.
Gyngard, F.
Presolar grains constitute remnants of stars that existed before the formation of the solar system.
In addition to providing direct information on the materials from which the solar system formed, these grains provide groundtruth information for models of stellar evolution and nucleosynthesis.
Here we report the insitu identification of two unique presolar graphite grains from the primitive meteorite LaPaz Icefield 031117. Based on these two graphite grains, we estimate a bulk presolar graphite abundance of 53+7 ppm in this meteorite. One of the grains (LAP141) is characterized by an enrichment in 12C and depletions in 33,34S, and contains a small iron sulfide subgrain, representing the first unambiguous identification of presolar iron sulfide. The other grain (LAP149) is extremely 13Crich and 15Npoor, with one of the lowest 12C/13C ratios observed among presolar grains. Comparison of its isotopic compositions with new stellar
nucleosynthesis and dust condensation models indicates an origin in the ejecta of a lowmass CO nova. Grain LAP149 is the first putative nova grain that quantitatively best matches nova model
predictions, providing the first strong evidence for graphite condensation in nova ejecta. Our discovery confirms that CO nova graphite and presolar iron sulfide contributed to the original building blocks of the solar system.

Constructing frozen Jacobian iterative methods for solving systems of nonlinear equations, associated with ODEs and PDEs using the homotopy
http://hdl.handle.net/2117/100357
Constructing frozen Jacobian iterative methods for solving systems of nonlinear equations, associated with ODEs and PDEs using the homotopy
Qasim, Uswah; Ali, Zulifgar; Ahmad, Fayyaz; Serra Capizzano, Stefano; Ullah, Malik Zaka; Asma, Mir
A homotopy method is presented for the construction of frozen Jacobian iterative methods. The frozen Jacobian iterative methods are attractive because the inversion of the Jacobian is performed in terms of LUfactorization only once, for a single instance of the iterative method. We embedded parameters in the iterative methods with the help of the homotopy method: the values of the parameters are determined in such a way that a better convergence rate is achieved. The proposed homotopy technique is general and has the ability to construct different families of iterative methods, for solving weakly nonlinear systems of equations. Further iterative methods are also proposed for solving general systems of nonlinear equations.
20170131T10:04:21Z
Qasim, Uswah
Ali, Zulifgar
Ahmad, Fayyaz
Serra Capizzano, Stefano
Ullah, Malik Zaka
Asma, Mir
A homotopy method is presented for the construction of frozen Jacobian iterative methods. The frozen Jacobian iterative methods are attractive because the inversion of the Jacobian is performed in terms of LUfactorization only once, for a single instance of the iterative method. We embedded parameters in the iterative methods with the help of the homotopy method: the values of the parameters are determined in such a way that a better convergence rate is achieved. The proposed homotopy technique is general and has the ability to construct different families of iterative methods, for solving weakly nonlinear systems of equations. Further iterative methods are also proposed for solving general systems of nonlinear equations.